This invention relates to a new process for the removal of arsenic impurities contained in distilled hydrofluoric acid, by means of oxidation with solid potassium permanganate (KMnO.sub.4) (100%) which is used as an oxidant and solid potassium bifluoride (KHF.sub.2) (100%) which is used as a donor of potassium ions to reduce the requisite amount of KMnO.sub.4 required. The oxidation step is followed by a purification step which includes rectification.
Acid grade fluorspar which is present in mineral deposits is rich in calcium fluoride (CaF.sub.2) and silicon dioxide (SiO.sub.2), but it also contains high levels of arsenic (500 ppm or more of As.sub.2 O.sub.3).
Arsenic is present in the fluorspar in the form of a solid and cannot economically be removed from the fluorspar.
In reaction with sulfuric acid, no less than approximately 90% of the arsenic present in fluorspar is converted into arsenic trifluoride (AsF.sub.3) and still remains in the gaseous hydrofluoric acid. In the condensation step, all of the arsenic trifluoride contained in the raw hydrofluoric acid condenses together with the hydrofluoric acid. Owing to the fact that the boiling point of arsenic trifluoride is nearly the same as the boiling point of hydrofluoric acid, practically speaking, the hydrofluoric acid/arsenic trifluoride cannot be separated by distillation.
A large portion (50%) of the hydrofluoric acid which is produced is used for the production of fluorocarbons. Arsenic has similar properties to those of antimony, which is used as a catalyst in the processing of fluorocarbons, and consequently it reduces the life of the antimony, thus adding to the cost of catalyst renewal and moreover to the problems of contamination. The generally accepted levels of arsenic in hydrofluoric acid are between 10 and 50 ppm. With arsenic levels of 500 ppm in the fluorspar, levels of up to 900 ppm can be expected in the hydrofluoric acid.
Arsenic trifluoride (AsF.sub.3) is a major part of the arsenic contained in hydrofluoric acid and cannot be separated therefrom by distillation.
Australian patent 266,930 discloses a process for removing arsenic from hydrofluoric acid. The process comprises treating the arsenic-containing hydrofluoric acid with an oxidizing agent, such as potassium permanganate, in the presence of alkali ions to form non-volatile pentavalent arsenic compounds. Example 4 on page 6 shows the use of potassium fluoride as the alkali ion source. This patent does not disclose the use of solid potassium bifluoride as the alkali metal source.
U.S. Pat. No. 3,166,379 teaches the purification of hydrofluoric acid involving the removal of impurities such as arsenic which comprises producing high boiling-point compounds by reacting hydrofluoric acid with an oxidizing agent such as potassium permanganate and an iodine, bromine or chlorine compound such as potassium iodide. At column 2, lines 71-72, this patent teaches away from using fluorine compounds.